So, made a foamboard/cardboard mock-up of the AC55 engine (without all the fins, and without the box sticking out the side, or the bottom "feet"). I think I made the diameter a few mm too short, but it appears that cutting the fins is enough to clear the drive axles! At worst, I'd maybe have to shave a few millimeters into the front casing of the AC55 engine, which seems like it will be ok, since it's pretty thick. As for the side to side clearance, the box in the back of the AC55 is just about touching the passenger side frame. Tight fit, hopefully this won't be an issue! I wonder if you can move that box somewhere else. I need a thin motor mount adapter, maybe 1.5 or so inches.
I wish the AC55 was just slightly smaller, it seems like a great motor, except for the size/weight issue. Size-wise, it's not THAT much bigger than the MES-DEA 200-250, about 1.5" of the diameter is the fins. It's about 100 pounds heavier though, at 235 pounds vs. 134 pounds.